Various image forming apparatuses are known that perform image formation by developing an electrostatic latent image formed on a photosensitive drum.
In such image forming apparatuses, an electrostatic latent image is formed on a surface of a photosensitive drum, and this image is then developed using toner. Toner is applied to the photosensitive drum by a developing roller that carries a thin toner layer thereon and is pressed against a surface of the photosensitive drum. After that, a visible image obtained by the development is transferred from the photosensitive drum onto a recording medium, such as a paper or plastic sheet, by a transfer roller.
In the image forming apparatus structure described above, if paper dust and/or toner (especially additives) remain on the photosensitive drum after the image is transferred onto the sheet, or if such foreign matter adheres to and/or builds up on the surface of the photosensitive drum, or if filming occurs in the photosensitive drum, image quality may be degraded and/or the life of the photosensitive drum may be shortened.
In an effort to resolve the above problem, for example, Japanese Laid-Open Patent Publication Nos. 2002-215002 and 11-52789 disclose a technique for removing filming or foreign matter adhered to a photosensitive drum therefrom by rubbing the photosensitive drum with a developing roller that is at a standstill. These publications are entirely incorporated herein by reference.
The developing roller is generally held at its end portions in its axial direction. Due to this structure, the developing roller may warp with respect to the axial direction when the developing roller is pressed against the photosensitive drum. More specifically, as shown in FIG. 12, a degree of contact between a developing roller 301 and a photosensitive drum 302 may be high at the vicinity of their end portions and lower at the vicinity of their middle portions in the axial direction (in the drawing, the warp of the developing roller 301 is exaggerated for the sake of illustration and clarity).
More specifically, a “nip” may be considered as a width of a portion where a developing roller and a photosensitive drum contact one another. In such known systems, the nip width may be different between the end portions and the middle portions of the contact area in the axial direction due to the warp of the developing roller 301. At the vicinity of the end portions of the developing roller 301 where the nip width is large, the pressing force of the developing roller 301 against the photosensitive drum 302 is strong, so that the amount of rubbing of the photosensitive drum 302 with the developing roller 301 is large. At the vicinity of the middle portion of the developing roller 301 where the nip width is smaller, however, the pressing force is weaker, so that the amount of rubbing of the photosensitive drum 302 with the developing roller 301 is smaller (see also FIG. 13).
For the reasons described above, when using the conventional developing roller 301, the surface of the photosensitive drum 302 can not be uniformly rubbed in the axial direction. As a result, variations may occur in image quality and/or the number of sheets that the photosensitive drum 302 can print, e.g., both over the long term and/or on an individual document (e.g., differences in print quality between the end portions and the middle portion of the photosensitive drum 302 in the axial direction may be observed on a single document).